234 J. W. GKOVES ON SECTIONS OF ANIMAL TISSUES. 



ing fluids. Alcohol should be used rather weak at first, then 

 stronger, till the material is hard enough to cut ; except when the 

 freezing microtome is to be employed. 



Miiller's solution is another most useful hardening agent, especially 

 for the retina and other nerve structures. Material should be kept 

 in it from three to six weeks. Its composition is — 



Bichromate of potash, 25 grams., or 2£ parts. 

 Sulphate of sodium, 10 „ 1 „ 



Water, 1,000 c, 100. 



The methods of hardening tissues by drying them is open to 

 many objections, among which may be mentioned the alteration of 

 structures and the difficulty of obtaining thin sections ; for, cut them 

 as thin as may be, they will swell when rendered moist again by 

 the subsequent processes. 



Softening fluids are usually only required for structures which are 

 naturally hard, due to the impregnation of lime salts, as bone and 

 teeth. For these acids are required, but as it is generally desirable to 

 retain in situ the soft parts connected with them, it is advisable to 

 use, as a softener of the inorganic matter, some fluid which will at 

 the same time harden all the soft or organic parts. For this purpose 

 a fluid consisting of 



Chromic acid, 1 grm., 



Nitric acid, 2 c.c. (this added last), 



Water, 200 c.c, 



is, perhaps, the best. In making it, care should be taken that the 

 chromic acid and water be first mixed, and the nitric acid added 

 subsequently. It is better to immerse specimens containing lime 

 salts in a one-eighth per cent, aqueous solution of chromic acid, or 

 a two per cent. sol. of Pot. Bichrom. for a few clays before placing 

 them in this mixture, as by that means the soft parts become more 

 perfectly hardened. A few agents, at the same time that they 

 harden, impart a colour to the tissues, thus, material which has been 

 soaked in chromic acid acquires a green tinge when placed in glycerine, 

 this, though handy for certain purposes, is not generally useful. 



Osmic acid, on the other hand, hardens, and, at the same time, 

 imparts a useful colour, for it selects all fatty matters and tinges 

 them black. After the material has been partially hardened in a 

 one-tenth to one per cent, aqueous solution of this acid, the process 

 may be completed in alcohol, though if left in acid solution for 12 to 

 48 hours it will be quite hard enough. When hard it should be 



